Now it seems more likely than ever that Democrats in the US Senate will not touch global warming in 2010 unless they can be assured of sizeable Republican support. Brown's election has also led to international concern that any failure to act by the US - the world's biggest historical polluter - would undermine attempts to seal a global deal.

However, Senator John Kerry, who is leading the push on climate change in the Senate, said he remained confident of getting broad support for a bill.

"The political atmosphere doesn't reduce the urgency of dealing with pollution and energy, and the surest way to increase the anger at Washington is to duck the issues that matter in peoples' lives. There's overwhelming public support and this can be a bipartisan issue," he said today . "This is the single best opportunity to create jobs, reduce pollution, and stop sending billions overseas for foreign oil from countries that would do us harm. Sell those arguments and you've got a winning issue."

The House of Representatives narrowly passed a climate change bill last June. But Senate Democrats had long calculated that - with the divisive fight over healthcare causing internal splits - their only hope of passing their own version of a climate bill was to win Republican support.

Kerry has been leading a tripartisan effort with Republican Lindsey Graham and independent Joe Lieberman to craft a bill that would pull support from at least a few Republicans. The troika has yet to produce a draft proposal, but there is anticipation of an expanded role for nuclear power, perhaps with more cheap government loans or streamlined regulations to get projects approved. There is also talk of offshore oil and gas drilling.

Some Senators have proposed limiting the scope of the bill, regulating only the biggest power plants, or perhaps encouraging renewable energy without laying the foundations of a carbon trading market. Other Democrats - who were opposed to a climate change bill even before the vote in Massachusetts - say the Senate is unlikely to move in 2010 without those compromises.

"It is my assessment that we likely will not do a climate change bill this year, but we will do energy," Byron Dorgan, a North Dakota Democrat who opposes action on climate change told reporters in a conference call yesterday. "I think it is more likely for us to turn to something that is bipartisan and will address the country's energy interest and begin to address specific policies on climate change."

Brown's victory robs the Democrats of their filibuster-proof 60-40 majority in the Senate. But it is not entirely clear the Senate's newest member would be an automatic no. In the excitement of the campaign, Brown cast himself as a climate change sceptic. "I think the globe is always heating and cooling," the Boston Globe quoted Brown as saying. "It's a natural way of ebb and flow. The thing that concerns me lately is some of the information I've heard about potential tampering with some of the information."

But as a Massachusetts state senator in 2008, Brown voted for a regional cap-and-trade regime, which is similar in concept to what the climate bill is proposing on a nationwide scale.

Outside the US, reaction to Brown's win suggested it made a global pact to fight global warming harder. Nick Mabey, head of the E3G climate thinktank in London, said without US action there were risks talks would stall. "We can't afford climate to be a dysfunctional regime like trade," like the inconclusive Doha round on freer world trade launched in 2001, he said.

"On the international front, China is constantly looking to the US on climate bills. This is definitely bad news. It doesn't bring new confidence to international negotiations," said Ailun Yang of Greenpeace in Beijing.

Shirish Sinha of WWF India said US action was essential but that "irrespective of what happens in US...it is on our self-interest to do something for climate change."